1. Field of the Invention
The present invention relates to a method for continuously tin-electroplating a metal strip.
2. Related Art Statement
As a method for continuously tin-electroplating a metal strip, there is known a method using an acidic tin-electroplating solution containing tin ions and an organic acid, which comprises the steps of: causing a DC electric current to flow between a metal strip and an anode arranged adjacent to said metal strip while passing said metal strip at a prescribed travelling speed through an acidic tin-electroplating solution containing tin ions and an organic acid, to form a tin-electroplating layer on at least one surface of said metal strip.
When divalent tin ions are present alone without combining with the other ions in an acidic tin-electroplating solution not containing an organic acid, the divalent tin ions are oxidized into tin oxides, during the tin-electroplating, by means of oxygen contained in the acidic tin-electroplating solution, causing the production of sludge in a considerable quantity. The thus produced sludge adheres onto the tin-electroplating layer formed on the surface of the metal strip, resulting in a lower quality of the tin-electroplating layer.
In contrast, when an acidic tin-electroplating solution containing an organic acid is used, divalent tin ions are combined with the organic acid during the tin-electroplating to form a complex, thereby inhibiting the production of sludge caused by the oxidation of divalent tin ions to avoid the above-mentioned inconvenience.
However, the above-mentioned method using the acidic tin-electroplating solution containing the tin ions and the organic acid has the following problems:
Formation of the tin-electroplating layer on the surface of the metal strip during the tin-electroplating comprises the following three steps: a movement step in which divalent tin ions in the acidic tin-electroplating solution move toward the surface of the metal strip (hereinafter referred to as the "material transfer process"); an electric charge transfer step on the surface of the metal strip (hereinafter referred to as the "electric charge transfer process"); and a crystallization step of tin on the surface of the metal strip (hereinafter referred to as the "crystallization process").
The divalent tin ions in the form of the complex are low in the transfer rate in the acidic tin-electroplating solution. Furthermore, the divalent tin ions in the form of the complex are dissociated into independent divalent tin ions upon the precipitation onto the surface of the metal strip. During the tin-electroplating, therefore, there is a portion of the acidic tin-electroplating solution, in which the divalent tin ion concentration becomes gradually lower from a position a prescribed distance apart from the surface of the metal strip toward the surface of the metal strip (such a portion being hereinafter referred to as the "diffusion layer of tin ions"). In the tin-electroplating using an acidic tin-electroplating solution containing tin ions and an organic acid, therefore, the material transfer process acts as a rate-determining step because of the presence of the diffusion layer of tin ions during the tin-electroplating, thus limiting the progress rate of all the other processes.
When carrying out the tin-electroplating with a high electric current density in a state in which the material transfer process acts as the rate-determining layer, i.e., in a state in which there is present the diffusion layer of tin-ions, the quality of the tin-electroplating layer formed on the surface of the metal strip is deteriorated as described below:
In order to prevent deterioration of the quality of the tin-electroplating layer as described above, it is necessary to decrease the electric current density per supply of electricity, and this results in a lower critical electric current density. A lower critical electric current density in turn leads to a smaller tin-plating weight per supply of electricity, and to more frequent supply of electricity in order to ensure a desired tin-plating weight. Since electricity is supplied only once for each tin-electroplating tank, more frequent supply of electricity requires more tin-electroplating tanks, and hence resulting in increased manufacturing and equipment costs.
As a means to solve the problems regarding the decrease in the critical electric current density caused by the presence of the diffusion layer of divalent tin ions, there is known a technology of providing an ejecting mechanism for ejecting an acidic tin-electroplating solution into the acidic tin-electroplating solution between the metal strip and the anode, and causing the acidic tin-electroplating solution between the metal strip and the anode to flow by means of the ejecting mechanism.
When conducting the tin-electroplating at such a high electric current density as over 100 A/dm.sup.2, however, it is necessary to eject the acidic tin-electroplating solution at a high rate of over about 5 to 10 m/second into the acidic tin-electroplating solution between the metal strip and the anode. This requires a large-scale and expensive ejecting mechanism. It is furthermore very difficult to control the ejecting rate of the tin-electroplating solution.
As means to solve the problems regarding the decrease in the critical electric current density caused by the presence of the diffusion layer of metal ions, many techniques using an ultrasonic vibration have been reported since the 1940s. T. Walker et al., for example, released their report as described below in the periodical "Galvano-Organo-Traitements de Surface", vol. 43, No. 449, November 1974, pages 1,009-1,014:
In order to solve the problems regarding the decrease in the critical electric current density caused by the presence of the diffusion layer of metal ions, the following horizontal-type electroplating apparatus using an ultrasonic vibration has been proposed:
A horizontal-type electroplating apparatus disclosed in Japanese Patent Provisional Publication No. 63-118,094 published on May 23, 1988, which comprises: a horizontal-type electroplating tank in which an electroplating solution is forcedly circulated; two anode plates each arranged above and below a metal strip at a prescribed distance therefrom and in parallel to said metal strip, which metal strip travels horizontally through said electroplating solution in said electroplating tank, each of said anode plates having a plurality of through-holes becoming gradual narrower toward said metal strip; and a plurality of ultrasonic vibrators each arranged outside said anode plate relative to said metal strip at a prescribed distance from said anode plate, for each of said through-holes (hereinafter referred to as the "prior art 1").
Accordingly to the above-mentioned prior art 1, the diffusion layer of metal ions existing in the electroplating solution adjacent to the surface of the metal strip can be removed by applying an ultrasonic vibration, having a frequency within a range of from 10 to 1,000 kHz and emitted form the ultrasonic vibrators toward the metal strip, through the through-holes to the electroplating solution between the metal strip and the anode plate.
However, the prior art 1 contains neither concrete disclosure nor suggestion regarding a means to solve the problems involved in the method for tin-electroplating using an acidic tin-electroplating solution containing tin ions and an organic acid.
For the purpose of solving the problems regarding the decrease in the critical electric current density caused by the presence of the diffusion layer of metal ions, the following method for electroplating using an ultrasonic vibration has been proposed:
A method for electroplating disclosed in Japanese Patent Provisional Publication No. 58-45,345 published on Mar. 16, 1983, which comprises the steps of: providing a nozzle adjacent to an end of an anode provided below a main roll; emitting an ultrasonic wave from said nozzle during the electroplating to apply an ultrasonic vibration to an electroplating solution between a travelling metal strip and said anode; and at the same time, ejecting an electroplating solution containing bubbles from said nozzle into the electroplating solution between said metal strip and said anode to cause the electroplating solution between said metal strip and said anode to flow (hereinafter referred to as the "prior art 2").
According to the above-mentioned prior art 2, it is possible to remove the diffusion layer of metal ions existing in the electroplating solution adjacent to the surface of the metal strip.
However, the prior art 2 contains neither concrete disclosure nor suggestion regarding a means to solve the problems involved in the method for tin-electroplating using an acidic tin-electroplating solution containing tin ions and an organic acid.
Under such circumstances, there is a strong demand for the development of a method for continuously tin-electroplating a metal strip, which, when causing a DC electric current to flow between a metal strip and an anode arranged adjacent to the metal strip while passing the metal strip through an acidic tin-electroplating solution containing tin ions and an organic acid, to form a tin-electroplating layer on at least one surface of the metal strip, enables to increase the critical electric current density by removing a diffusion layer of tin ions existing in the acidic tin-electroplating solution adjacent to the surface of the metal strip, so as to form a tin-electroplating layer excellent in quality on the surface of the metal strip at a high electric current density, and furthermore, to reduce the number of tin-electroplating tanks by reducing the frequency of supply of electricity, thereby permitting reduction of costs for manufacture and equipment, but such a method has not as yet been proposed.